how to use displacement maps on Eevee Blender 2.8
It's working as usual, like this:

I have a subdivided plane here, then i went into the modifiers tab, added a displace modifier, then you click new texture, after this, go to the texture tab:

In here you can either choose an existing image to displace your mesh or a noise texture already available:

Then, when you leave edit mode, you should have your mesh with the displacement that you chose:

In this case, Voronoi. You should always download the latest version of blender 2.8 in the experimental tab on the blender.org website, maybe the version that you're using didn't have the displacement availabe :)